Novel rare-earth doped fluorides compositions

ABSTRACT

The present invention is directed to rare-earth doped solid state solutions of alkaline earth fluorides having novel luminescence properties, and to a process for preparing them. The invention is useful as identifying markers on articles. Other uses include phosphors for plasma displays, optical frequency multipliers, optical amplifiers and the like.

FIELD OF THE INVENTION

The present invention is directed to rare-earth doped alkaline earthfluorides having novel luminescence properties, and to a process forpreparing them. The invention is useful as identifying markers onarticles. Other uses include phosphors for plasma displays, opticalfrequency multipliers, optical amplifiers and the like.

BACKGROUND OF THE INVENTION

Luminescent rare-earth doped alkaline-earth fluorides have long beenknown, and have been employed for numerous purposes such asscintillation detectors and laser materials. CaF₂ doped with suchrare-earth species as Eu⁺³, Er⁺³, Tb⁺³ are well-known compositions. Itis well-known that a rare-earth doped alkaline earth fluoride willexhibit luminescence when exposed to ultraviolet light.

Each rare-earth element when incorporated into an alkaline earth hostlattice such as CaF₂ exhibits a characteristic excitation spectrum; see,for example, FIG. 1 (101), and a characteristic emission or luminescencespectrum that depends upon the excitation wavelength employed; see, forexample, FIG. 1 (102). The excitation spectrum is determined bymonitoring the luminescence intensity at one wavelength while thespecimen is illuminated over a range of wavelengths. The luminescencespectrum is determined by illuminating the specimen at a singlewavelength corresponding to a peak in the excitation spectrum anddetermining the luminescence spectrum by scanning a detector over arange of wavelengths.

As shown in the figures, each such spectrum consists of a plurality ofpeaks at different wavelengths of light. The wavelengths at which thepeaks occur are characteristic of each rare-earth element. No tworare-earth elements exhibit the same excitation or emission spectra;that is, the peaks in their spectra do not in general arise at the samewavelengths. To obtain luminescence, the rare-earth element must beexcited by a light source that emits light at a wavelength correspondingto the location of one of the peaks in the excitation spectrum thereof.In general, the peaks in any one spectrum of rare-earth elements differfrom one another in height or intensity, these differences in intensitybeing characteristic of the rare-earth element under particularconditions of measurement. These and related matters are allwell-documented in the art. See for example, Martin et al., AtomicEnergy Levels—the Rare-Earth Elements, U.S. Department of Commerce,National Bureau of Standards (1978).

Copending application 60/687,646 discloses a room temperature aqueoussolution based method for preparing rare-earth doped alkaline earthfluoride nanoparticles.

Sarma et al., Solid State Ionics 42, 227 (1990) discloses solid statesolutions of CaF₂ and SrF₂. No mention is made of rare-earth doping.

Federov et al., Doklady Akademii Nauk. 369(2):217-219, 1999, disclosessolid solutions consisting of a series of 10 mm diameter and 50 mm longsingle crystals of (Ca_(1−y)Sr_(y))_(1−x)Nd_(x)F_(2+x) grown by theBridgman-Stockbarger method by crystallization from the melt.

Considerable effort in the art is being directed towards developingluminescent compositions for use as identifying marks on commercialgoods, including packages, manufactured articles, and even money. Oneidea is to place an identifying mark on a manufactured article whichwill attest to its authenticity in the face of rampant piracy on aglobal scale. The mark is ideally invisible until inquiry is made usinga particular wavelength of light which then stimulates luminescence witha characteristic spectrum.

A simple luminescent security mark may itself be easy to counterfeit.The present invention provides a family of novel rare-earth-dopedalkaline earth fluorides, and a process for preparing them, that arecharacterized by unique luminescence peak intensity ratios, making itextraordinarily difficult to counterfeit security marks comprising thesecompositions.

SUMMARY OF THE INVENTION

The present invention provides a composition comprising arare-earth-doped solid-state solution of alkaline earth fluoridesrepresented by the chemical formula

RE_(x)(Ca_(a)Sr_(b)Ba_(c))_(1−x)F_(2+x−2y)O_(y)

wherein RE represents a three-valent rare-earth element, 0.005≦×≦0.20,and 0≦y≦0.2, a+b+c=1, with the proviso at least two of a, b, and c arenot equal to zero; the composition exhibiting a luminescence spectrumhaving a plurality of luminescence peaks at characteristic wavelengths,at least one pair of the luminescence peaks exhibiting an intensityratio with respect to one another that differs by at least 5% from thecorresponding intensity ratio of a corresponding reference composition.

The present invention further provides a process comprising combining anaqueous solution of an ammonium fluoride or hydrogen fluoride, ormixtures thereof, with one or more aqueous solutions of the salts of atleast two alkaline earth metals, and an aqueous solution of a saltcomprising a 3-valent rare earth metal cation, the amount of therare-earth metal cation being in the range of 0.5 to 20 mol-% of themolar concentration of the total alkaline earth metal cation content,thereby forming a reaction mixture from which is formed a precipitate ofa rare-earth doped solid state solution of alkaline earth fluoridesrepresented by the formula

RE_(x)(Ca_(a)Sr_(b)Ba_(c))_(1−x)F_(2+x−2y)O_(y)

wherein RE represents a three-valent rare-earth element, 0.005≦×≦0.20,and 0≦y≦0.2, a+b+c=1, with the proviso at least two of a, b, and c arenot equal to zero; the rare-earth doped multi-valent metal fluoridebeing characterized by an aqueous solubility of less than 0.1 g/100 g ofwater.

Further provided is a process comprising heating in the presence ofoxygen a composition comprising a rare-earth-doped solid-state solutionof alkaline earth fluorides represented by the chemical formula

RE_(x)(Ca_(a)Sr_(b)Ba_(c))_(1−x)F_(2+x−2y)O_(y)

wherein RE represents a three-valent rare-earth element, 0.005≦×≦0.20,and 0≦y≦0.2, a+b+c=1, with the proviso at least two of a, b, and c arenot equal to zero; to a maximum temperature in the range of 200° C. to1000° C. followed by cooling, with the proviso that the composition soheated has not previously been exposed to the maximum temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an excitation spectrum ofEu_(0.05)Ca_(0.95)F_(2.05−2y)O_(y).observed at 591 nm and a luminescencespectrum of Eu_(0.05)Ca_(0.95)F_(2.05−2y)O_(y) excited at 394 nm.

FIG. 2 illustrates a X-ray diffraction pattern of a precipitatedcomposition of Example 2.

FIGS. 3A and 3B illustrate graphs of lattice parameters.

DETAILED DESCRIPTION

The present invention provides a composition comprising arare-earth-doped solid-state solution of alkaline earth fluoridesrepresented by the chemical formula

RE_(x)(Ca_(a)Sr_(b)Ba_(c))_(1−x)F_(2+x−2y)O_(y)

wherein RE represents a three-valent rare-earth element, 0.005≦×≦0.20,and 0≦y≦0.2, a+b+c=1, with the proviso at least two of a, b, and c arenot equal to zero; the composition exhibiting a luminescence spectrumhaving a plurality of luminescence peaks at characteristic wavelengths,at least one pair of the luminescence peaks exhibiting an intensityratio with respect to one another that differs by at least 5% from thecorresponding intensity ratio of a corresponding reference composition.

For the purpose of the present invention the term “solid state solution”is employed to refer to a composition such as but not limited toSr/CaF₂:EuF₃, that forms a single crystalline phase as indicated byx-ray diffraction (XRD) analysis whereas a simple mixture of, e.g., aSrF₂: EuF₃ and a CaF₂:EuF₃ is shown by XRD to consist of multiplecrystalline phases. XRD of the crystalline solid solutions of theinvention reveals a well-defined crystallographic lattice parameter thatis observed to vary linearly with mole fraction of Sr (x) inCa_(0.98−x)Sr_(x)F₂:Eu_(0.02) as shown in the specific embodimentsinfra. This linear dependency is known to be a characteristic of solidsolutions based upon Vegard's Law (see the descriptions of solidsolutions and Vegard's Law given, for example, in the standard textStructural Inorganic Chemistry by A. F. Wells, Oxford University Press,1962, third edition). In contrast, the XRD pattern for a simple mixtureof, e.g., a SrF₂: EuF₃ and a CaF₂:EuF₃ that is a simple linearsuperposition of the XRD patterns of separate constituent phases, eachweighted by its respective volume fraction.

The compositions of the present invention may conveniently be preparedaccording to the precipitation process and, if desired, the heatingprocess of the invention, described, infra. However, the composition isnot limited in scope to any particular means of preparation. For thepurposes of the present invention, the process by which the compositionis synthesized shall be known as the “precipitation process.” Anyembodiment of the composition that has not been exposed to a temperatureabove 100° C. shall be referred to as an “as-precipitated” embodimentregardless of whether that embodiment was actually prepared byprecipitation.

The composition of the invention has many embodiments that differ fromone another, inter alia, by virtue of the particular rare-earth, and theparticular alkaline earth cations incorporated therein, as well as bythe relative amounts thereof, that is, by the values of x, a, b, and c.To each embodiment there corresponds a so-called “referencecomposition.” The reference composition is a solid state solutionconsisting of the same rare-earth and alkaline earths in the samerelative amounts as the embodiment to which it corresponds; that is, RE,x, a, b, and c are the same as in the embodiment to which itcorresponds. However, unlike an embodiment, that is prepared accordingto the process, the reference composition corresponding thereto isprepared by crystallization from the melt in the manner of Federov, op.cit. Each embodiment, and each reference composition correspondingthereto, is characterized by a luminescence spectrum having a pluralityof luminescence peaks at characteristic wavelengths. Any pair of theplurality of luminescence peaks is characterized by the ratio of theintensities (or heights) thereof. According to the present invention,for each embodiment, there is at least one pair of the plurality ofpeaks whereof the ratio of the intensities (or intensity ratio) differsby at least 5% from the intensity ratio of peaks at the same wavelengthsin the luminescence spectrum of the corresponding reference composition.

The rare-earths suitable for the practice of the invention include allthe members of the Lanthanide series in the periodic table of theelements with the exception of promethium and lutetium. The rare-earthelements are all in the +3-valent state. Eu+3, Er+3, and Tb+3 arepreferred. In a further embodiment 0.01≦×≦0.10.

In another embodiment, one of a, b, or c=0. In a still furtherembodiment, a=0.01 to 0.99, b=0.99 to 0.01, and c=0. In a furtherembodiment, a=0.25 to 0.75 and b=0.75 to 0.25, while c=0.

In a further embodiment, RE is Eu+3, Er⁺³, or Tb⁺³, 0.01≦×≦0.10, a=0.01to 0.99, b=0.99 to 0.01, and c=0, In a still further embodiment the rareearth-doped solid state solution of alkaline earth fluorides isrepresented by the chemical formulaEu_(0.02)(Ca_(0.50)Sr_(0.50))_(0.98)F_(2.02−2y)O_(y) where 0≦y≦0.2.

The composition is not limited to any particular method by which it isprepared.

The present invention further provides a process comprising combining anaqueous solution of an ammonium fluoride, hydrogen fluoride, or mixturesthereof, with one or more aqueous solutions of the salts of at least twoalkaline earth metals, and an aqueous solution of a salt comprising a3-valent rare earth metal cation, the amount of the rare-earth metalcation being in the range of 0.5 to 20 mol-% of the molar concentrationof the total alkaline earth metal cation content, thereby forming areaction mixture from which is formed a precipitate of a rare-earthdoped solid state solution of alkaline earth fluorides represented bythe formula

RE_(x)(Ca_(a)Sr_(b)Ba_(c))_(1−x)F_(2+x−2y)O_(y)

wherein RE represents a three-valent rare-earth element, 0.005≦×≦0.20,and 0≦y≦0.2, a+b+c=1, with the proviso at least two of a,b, and c arenot equal to zero; the rare-earth doped multi-valent metal fluoridebeing characterized by an aqueous solubility of less than 0.1 g/100 g ofwater.

The reaction in aqueous solution of the soluble fluoride with thesoluble alkaline earth salts and rare earth salt is very rapid.Precipitation occurs so quickly in the process of the invention thatthere is little time for crystal growth after nucleation except inhighly dilute solution and low supersaturation.

The particles produced according to the present invention comprise acrystalline or semi-crystalline host material and a dopant. The hostmaterial is a solid state solution of at least two alkaline earthfluorides characterized by an aqueous solubility of less than 0.1 g/100g of water. The dopant is a three-valent rare-earth cation whichoccupies specific lattice sites in the crystalline structure of the hostmaterial.

According to the present invention an aqueous solution of ammoniumfluoride is combined with one or more aqueous solutions of the salts ofat least two alkaline earth metals, and an aqueous solution of a rareearth metal salt. The aqueous solubility of the resulting rare-earthdoped solid state solution of two or more alkaline earth fluorides isless than 0.1 g/100 g at room temperature.

The term “rare-earth” refers to members of the Lanthanide Series in theperiodic table, namely La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,and Yb.

Preferred anions for a soluble alkaline earth metal salt suitable forthe process hereof include chloride, nitrate, sulphate, hydroxide,acetate, carbonates, phosphates, bromides, and hydrates thereof.

The process can be employed to make both nano-scale particles andmicro-scale particles, depending upon the reaction conditions. For thepurpose of the present invention, the term “nano-scale” shall beunderstood to refer to a batch of particles of which fewer than 50%,preferably fewer than 90%, of the particles by weight are trapped in a200 nm filter. It has been found in the practice of the invention that a0.2 micrometer Zapcap-CR chemically resistant bottle top filteravailable from Schleicher & Schueel Microscience, is satisfactory forthis determination.

For the purpose of the present invention, the term micro-scale shall beunderstood to refer to a batch of particles of which at least 50%,preferably at least 90%, of the particles by weight are trapped in a 500nm filter. Particles to be micro-sized shall be further characterized inthat fewer than 50%, preferably fewer than 90%, of the particles byweight are trapped in a 200 micrometer filter.

Three processes are involved in the precipitation of a solid productfrom a homogenous reaction solution. (1) chemical reactions that producesupersaturation, (2) nucleation of particles and (3) and growth ofparticles. For fast reactions, such as occur herein, small particles areproduced when there is a localized high concentration of thecrystallizing species in solution and high supersaturation, whichresults in high nucleation rates, high nuclei densities, and low growthrates. Large particles are produced by reducing the local solutionsupersaturation, which decreases the nucleation rate and increases thegrowth rate.

From a processing standpoint, final particle size can be influenced bycontrolling initial reactant concentrations, crystallizing speciesconcentration (supersaturation) and mixing conditions.

It was observed in the practice of the present invention that increasingthe concentration of the rare-earth dopant decreases the size of theparticle produced according to the present invention. As a generalguideline, preparation of nano-scale particles is beneficiallyaccomplished by employing reactant concentrations of >0.01N, preferablyin the range of 0.1N to 0.8N, while preparation of micro-scale particlesis beneficially accomplished by employing reactant concentrations of<0.01N. Nano-scale particles may beneficially be prepared by directmixing of the precursor solutions as in a T-mixer or by some other formof direct mixing. In these cases the local supersaturation is highresulting in high nucleation rates, low growth rates and nano-scaleparticles. While micro-scale particles can also be prepared by directmixing of highly dilute solutions, it is more convenient to combineabout 1N solutions of the reactants in a well-stirred aqueous bath thatprovides a dilution factor of ca. 100-200 times—for example combining 1liter of 2N of each alkaline earth chloride, and the appropriate amountof EuCl₃, and NH₄F in 120 liters of well-stirred water has been found tobe satisfactory for preparing micro-scale particles. In these cases thelocal supersaturation is low resulting in low nucleation rates, highgrowth rates and micron size scale particles. It is important to stirthe reaction vessel to effectively reduce the local supersaturation.

The process of the present invention is also applicable to thepreparation of nano-scale and micro-scale mixed fluoride salts which areundoped with rare earth. For instance, the process of the presentinvention can be used to prepare nano-scale and micro-scale particles ofundoped SrCaF₂.

Soluble salt starting materials need only be soluble enough to formaqueous solutions of the desired concentrations for the purposes of thepresent invention. From the standpoint of the present invention, a saltis to be aqueously soluble if a solution of the desired concentrationcan be formed from it.

For the production of nano-scale particles, it is convenient to combinethe reactants in a T-mixer on a continuous or semi-continuous basis.Reaction is essentially instantaneous, with nano-particulate precipitateforming in the output leg of the T as the reaction stream flows into thecollector vessel. For production of micro-scale particles, the highlydiluted ingredients, with concentrations of <0.01N, may need to beallowed to stand and react while being stirred for about 30 minutes. ThepH of the reaction mixture is preferably maintained close to neutral,but a pH range from about 1 to 11 is acceptable.

Following reaction and product precipitation, the product may beconveniently separated by centrifugation and decanting of thesupernatant liquid. The isolated “wet cake” so produced may then beredispersed in water (or organic solvents by a solvent exchange process)by mixing with liquid and subjecting the mixture to ultrasonic agitationfor a period of 5-30 minutes. The dispersed particles are then in a formwell-suited to use in coatings and the like. For dispersion in non-polarsolvents, it may be required to combine the particles produced withsurfactants, as taught in the art.

Other suitable methods of separating the precipitate include ionexchange, dialysis or electrodialysis which substantially eliminates allsalts produced in the process. Further methods, to separate andconcentrate the sample, include evaporation of water, centrifugation,ultrafiltration, electrodecantation. A preferred procedure is to employion exchange resins to remove soluble salt residues followed byevaporation to concentrate the colloidal sol produced in the process.

It is preferred that the particles prepared in the process of theinvention be subject to water washing in order to remove any residualwater soluble starting materials. Dispersing in water followed bycentrifugation is one effective method.

The resulting particles exhibit luminescence when subject to suitableoptical excitation. It has been found that thermal post-treatment fromabout 200° C. to 1000° C. may alter luminescence intensity or lifetime.

When a composition comprising a rare-earth-doped solid-state solution ofalkaline earth fluorides represented by the chemical formula

RE_(x)(Ca_(a)Sr_(b)Ba_(c))_(1−x)F_(2+x−2y)O_(y)

wherein RE represents a three-valent rare-earth element, 0.005≦×≦0.20,and 0≦y≦0.2, a+b+c=1, with the proviso at least two of a, b, and c arenot equal to zero, that has been synthesized at a temperature belowabout 100° C., is heated in the presence of oxygen in the temperaturerange from 200-1000° C., it gives rise to a family of novelrare-earth-doped alkaline earth fluorides that differ from one anotherin their luminescence peak intensity ratios. Each member of the familyexhibits a luminescence spectrum having a plurality of luminescencepeaks at characteristic wavelengths. At least one pair of theluminescence peaks exhibits an intensity ratio that differs by at least5% from the corresponding intensity ratio of the corresponding referencecomposition, described supra.

While not limiting, it is found in the heating process of the invention,that upon heating to a temperature in the range of 200-1000° C. in thepresence of oxygen the value of y is observed to increase.

Every member of each such family of compositions exhibits a luminescencespectrum having a plurality of luminescence peaks at characteristicwavelengths. For each family of compositions according to the presentinvention, there exists at least one pair of the peaks, the relativeintensities of which change depending upon the temperature/time profileto which the as-precipitated composition, described supra, is subjected.The heated compositions are characterized by at least one peak intensityratio that differs by at least 5% from the corresponding peak intensityratio of the corresponding reference composition as well as from theas-precipitated composition. The term “corresponding peak intensityratio” refers to the peak intensity ratio of the same peaks in thecorresponding reference composition as that of the peaks in thecomposition of the invention to which peak intensity ratio is beingcompared.

For the purposes of the present invention, a family of compositions isone in which all members thereof have the same rare-earth element at thesame concentration, x; the same alkaline earth elements at the sameconcentrations, a(1-x), b(1-x), c(1-x); fluorine and oxygen, and whereinmembers are usually differentiated from one another by the value of y aswell as by the relative peak intensity ratio of at least one pair ofluminescence peaks.

In one embodiment of the heating process of the invention a compoundrepresented by the chemical formulaEu_(0.02)Ca_(0.49)Sr_(0.49)F_(2.02−2y)O_(y) wherein 0≦y≦0.2 issynthesized at room temperature in a fully aqueous solution describedsupra. The as-synthesized composition is then subject to heating in airto several temperatures between 200 and 1000° C. to yield a family ofcompositions wherein 0≦y/x≦1.

In another embodiment of the process of the invention a compoundrepresented by the chemical formulaEu_(0.02)Ca_(0.74)Sr_(0.24)F_(2.02−2y)O_(y) wherein y/x<0.05 issynthesized at room temperature in a fully aqueous solution described incopending U.S. application 60/687,646. The as-synthesized composition isthen subject to heating in air to several temperatures between 200 and1000° C. thereby producing the family of compositions wherein 0≦y/x≦1.

Other embodiments include but are not limited to applying the sameprocess to similarly precipitated compositions such asEu_(0.02)Ca_(0.24)Sr_(0.74)F_(2.022y)O_(y) wherein 0≦y≦0.2.

The as-precipitated compounds hereof are found to generally containsmall amounts of oxygen which may arise from a variety of adventitioussources. However, the concentration of oxygen in the as-precipitatedcompounds is found in the practice of the invention to be small, withy/x<0.05. On the other hand, the heated compositions of the inventionexhibit considerably larger oxygen concentrations.

The specific wavelengths of the emission peaks making up at least onepeak intensity ratio of any particular composition of the inventiondepend upon the specific rare-earth element employed, and to a lesserdegree to the host lattice—that is, the specific alkaline earthfluoride. However, all the compositions of the invention exhibit thesame differentiating characteristic regarding peak intensity ratiochanges.

The heating process of the invention can be performed in a series ofheating steps as well as in a single heating step. For example, acomposition as-precipitated can first be heated to, e.g., 300° C.,cooled, and at a later time further heated to, e.g., 500° C. The sampleheated to 500° C. can then again be cooled and at a later time heatedfurther to a still higher temperature.

Regardless of the heating protocol followed, there is no specificminimum duration of heating except that the duration of heating of aparticular composition at a particular temperature must be of sufficientlength to cause a change of at least 5% in the peak intensity ratio ofat least one pair of peaks in the luminescence spectrum.

The particular means employed for heating is not material to theoperability of the invention. Suitable means for heating include but arenot limited to pressure vessel heating of an aqueous dispersion(so-called hydrothermal heating), electrical resistance furnaces, oilbaths, electrically heated crucibles, liquid metal baths, lasers, radiofrequency induction furnaces, microwave cavities, gas fired furnaces,oil fired furnaces, solar furnaces. Preferred is an electricalresistance furnace. Typically, when heated in a bath, the as-synthesizedpowder is sealed in a pressure vessel of sufficient volume to leave ahead-space comprising oxygen followed by immersion of the heated tubeinto the heating bath. When the as synthesized powder is subject to ovenor furnace heating it can be heated in an open crucible.

It has been found satisfactory in the heating process of the inventionto heat the composition suitable for the practice of the inventiongradually to the desired final temperature such as by placing thespecimen to be heated into a furnace at room temperature, and thenheating to the desired end-point at a rate of 2-10° C./minute,preferably 4-6° C./min.

Heating is effected in the presence of oxygen. There are many potentialsources for the oxygen. Heating can be effected in the air, or in anoxygen atmosphere. It is also possible for oxygen to be devolved fromspecies employed or derived from the synthesis environment such asnitrates or hydroxyls. It is believed that even small amounts of oxygencontamination can be sufficient to effect the process.

It was found that the as-precipitated particles will undergo some degreeof sintering or agglomeration during heating, particularly at the highertemperatures in the temperature range. Depending upon the particularexigencies of the end use intended, it can be desirable to subject theproduct of the process to a means for comminution to smaller size. Mediamilling is one such method for reducing and/or homogenizing the particlesize. Numerous other methods are known in the art.

The rare-earth-doped solid state solutions of alkaline earth fluoridescan be combined with other ingredients to form compositions suitable foruse as coatings or inks. In one embodiment, a composition isincorporated into an ink composition suitable for printing. In anotherembodiment, the composition is incorporated into a paint compositionwhich can be applied by any method known in the art including bybrushing, rolling, or spraying.

Numerous chemical formulations are known in the art for preparing inks,paints, and other coating compositions. Every such composition in theart that contains inorganic pigments in particulate form can be employedto formulate an ink, paint, or other coating composition with thecomposition of the invention serving as the pigment. The composition ofthe invention may serve as the only pigment, or it may be combined withother pigments and particulate matter such as is known in the art ofinks and coatings.

In one formulation, an embodiment of the composition is incorporatedinto an ink or coating with no other pigment, thereby resulting in aluminescent coating that after application to the surface of an articleis largely invisible to the eye until subject to UV excitation ofluminescence.

The invention is further described in the following specificembodiments, but is not limited thereto.

EXAMPLES Luminescence Spectra

The luminescence spectra in the examples below were determined using aJobin-Yvon Spex Fluorolog spectrofluorometer. A 450 W Xe lamp was usedas the excitation source. Gratings blazed at 330 nm with 1200 grooves/mmwere used in the excitation monochromator. Gratings blazed at 500 nmwith 1200 grooves/mm were used in the emission monochromator. A drypowder sample was loaded into a 15 mm long by 5 mm diameter quartz tube.The powder was tamped down to provide a smooth sample surface and theends of the tube were sealed either with epoxy or cotton plugs. Thesample tube was then loaded in a sample holder designed to hold thesesmall tubes. Sample luminescence was measured from the front face of thetube, with an angle of 15° between the excitation and emission beams. A400 nm low-pass filter was used to prevent the primary excitation beamin second or higher order of the emission monochromator from interferingwith the results. Excitation and emission spectrometer bandwidths were 1nm; spectrum step size was 1 nm; integration time was 0.1 second perdata point. Data was corrected for the excitation Xe lamp intensity.

XRD

XRD data was obtained at DND-CAT at the Advanced Photon Source, ArgonneNational Laboratory, Argonne, Ill. The synchrotron beamline 5-ID-Bincludes an insertion device for high brilliance and operates at a fixedx-ray energy of 17 keV (wavelength of 0.7 Å) with a beam size of 0.5mm×0.5 mm. Data were collected in Bragg-Brentano scattering geometryusing a 2-circle Huber diffractometer over a two-theta (twice thescattering angle) range of 11-40 degrees. The experimental designincluded a Ge analyzer crystal for high angular resolution. The anglestep size was between 0.001-0.015 degree two-theta, depending upon thecrystallinity of the sample. Count time was one second per data point.The sample was loaded in a 1 mm diameter glass capillary that wasrotated about its long axis at 1 degree/sec during data collection toimprove powder averaging

Examples 1-3

In the amounts shown in Table 1 CaCl₂.2H₂O (Sigma-Aldrich, 99.9%),SrCl₂.6H₂O (Sigma-Aldrich, 99.9%) and EuCl₃.6H₂O (Sigma-Aldrich, 99.9%)were stirred into 150 ml of deionized water in a polycarbonateErlenmeyer flask for about 5 minutes to ensure dissolution of thesolids.

Separately, NH₄F(Alfa Aesar, ASC reagent 99%) was dissolved in theamounts shown in Table 1 by stirring into a further 150 ml aliquot ofdeionized water for about 5 minutes to ensure dissolution of the solids.

The prepared solutions were simultaneously but separately fed by aperistaltic pump at 10 ml/min through silicone rubber tubing into thetwo arms of a plastic tee (T). Teflon® tubing ran from the leg or outputbranch of the T into the product flask. A precipitate formed within theoutput branch of the T immediately beyond the point at which the twostreams merged, forming a suspension in the water. The suspension formedwas discharged into the product flask. During the discharge the flaskcontaining the product suspension was stirred by magnetic stirring.After discharge was complete, the suspension was held static for about24 hrs at ambient temperature. The resulting suspension was thencentrifuged (Sorvall RC5C, Thermo Electron Corp.) at a relativecentrifugal force of 9500×g for 40 min, and the supernatant (containingsoluble salts) decanted and discarded. The residue was redispersed in afresh aliquot of about 300 ml of deionized water using ultrasonicagitation (Sonics and Materials, Inc, Danbury, Conn.) at 50 W/cm². Theresulting dispersion was again centrifuged and the supernatant againdecanted and discarded.

The washed as precipitated powder residue was dried in a laboratorydrying oven at 60° C. in air for 24 hrs to form 15.67 g of a dry powdercompact. The oven dried powder compact was then hand-ground in a mortarand pestle to from a uniform dry powder.

The X-ray diffraction pattern of the as precipitated composition ofExample 2, Eu_(0.02)Sr_(0.49)Ca_(0.49)F_(2.02−2y)O_(y), was determined,and is shown as the solid line in FIG. 2.

The luminescence spectrum of the as-precipitated material was determinedby the procedure described supra, with the 589 nm/610 nm peak intensityratios of each shown in Table 1.

Examples 4-6

A 15 g aliquot of each of the washed as precipitated powders fromExamples 1-3 was redispersed in about 100 ml of deionized water usingultrasonic agitation. The washed 100 ml suspension was placed in a 100ml screw cap flexible-walled Teflon® bottle. The bottle was filled rightto the brim before placing the cap on in order to exclude air. TheTeflon® container containing the suspension was placed in a stainlesssteel pressure reactor (filled with water) and heated for 6 hrs at 245°C. at a saturated vapor pressure of 568 psi. The resultinghydrothermally treated suspension was then centrifuged and decanted asdescribed above. The wet powder residue was dried in a laboratory dryingoven at 60° C. in air for 24 hrs to form a dry powder compact. The ovendried powder compact was then hand-ground in a mortar and pestle to froma uniform dry powder.

FIG. 3 a shows a graph of the lattice parameter, a, determined byordinary means known to one of skill in the art from the XRD analysis,of the product of the hydrothermal heating step of the process for eachof the three compositions of Examples 1-3 as well as the pure Sr and Cacompositions of the comparative experiments. The linearity is indicativeof a solid state solution.

The luminescence spectrum was determined as described supra. Peaks at589 nm and 610 nm were discerned. Table 1 shows the 589/610 peakintensity ratios for the solid state solutions of Examples 4-6.

Examples 7-9

Fresh powder aliquots of the as-precipitated specimens prepared inExamples 1-3 were each placed in a covered alumina crucible which inturn was placed into a Fisher Isotemp Programmable Ashing Furnace, Model497, at room temperature. The specimen was then heated in air at aprogrammed rate of 5° C./min to the 900° C., held isothermally for 1 hr,followed by cooling at 5+C./min to room temperature. The oven firedpowder compact was then hand-ground in a mortar and pestle to from auniform dry powder.

Luminescence of each was determined as described supra, and results areshown in Table 1.

FIG. 3 b shows a graph of the lattice parameter, a, determined byordinary means known to one of skill in the art from the XRD analysis,of the product of the 900° C. heating step of the process for each ofthe three compositions of Examples 4-6 as well as the pure Sr and Cacompositions of the comparative experiments. The linearity is indicativeof a solid state solution.

Example 10

7.2 gr of CaC1 ₂.2H₂O, 13.06 gr of SrCl₂.6H₂O and 0.73 gr EuCl₃.6H₂Owere stirred into 1000 ml of deionized water in a polycarbonateErlenmeyer flask for about 5 minutes to ensure dissolution of thesolids.

Separately, 7.2 gr NH₄F was dissolved a further 1000 ml aliquot ofdeionized water for about 5 minutes to ensure dissolution of the solids.

A separate reaction vessel was charged with 1000 ml of pure deionizedwater. The reaction vessel with pure water was mechanically stirred witha 1.5″ diameter impeller positioned near the bottom of the vessel.

The prepared salt solutions were simultaneously but separately fed by aperistaltic pump at 10 ml/min through silicone rubber tubing into thestirred reaction vessel containing pure water. The feed tubes werepositioned on either side of the reaction vessel with the discharge endnear the impeller in order to get good mixing. A white precipitateformed in the reaction vessel on discharge of the two salt solutionscreating a particle-water suspension. During the discharge the reactionvessel containing the product suspension was continuously stirred. Afterdischarge was complete, the suspension was held static for about 24 hrsat ambient temperature. Water was removed from the resulting suspensionby filtering using a 0.5 micrometer filter (Zapcap-CR chemicallyresistant bottle top filter available from Schleicher & SchueelMicroscience) where the suspended particles were captured on the filtermedia. The wet filter cake was then washed 2× by adding clean water tothe filter cup and repeating the filter procedure.

The washed as precipitated powder residue was dried in a laboratorydrying oven at 60° C. in air for 24 hrs to form 15.67 g of a dry powdercompact. The oven dried powder compact was then hand-ground in a mortarand pestle to from a uniform dry powder.

Comparative Examples A, B, and C

Each of two Mixture Components, Eu_(0.02)Ca_(0.98)F_(2.02−2y)O_(y) shownas MC1 in Table 2, and Eu_(0.02)Sr_(0.98)F_(2.02−2y)O_(y) shown as MC2in Table 2, were prepared by the method of Examples 1-3 except that ineach case only one alkaline earth chloride was employed in thepreparation of each of the mixture components, as shown in Table 2. InFIG. 2 are the individual x-ray diffraction patters ofEu_(0.02)Sr_(0.98)F_(2.02−2y)O_(y) andEu_(0.02)Ca_(0.98)F_(2.02−2y)O_(y), represented as broken lines.

One 15 g aliquot of each Mixture Component was separately hydrothermallyheated as described above in Examples 4-6. Following heating and drying,the dried powders of the hydrothermally heated Mixture Components werecombined in the molar ratios shown in Table 3 and then physically mixedtogether by grinding by hand in a mortar and pestle to form a uniformpowder mixture.

Luminescence data was obtained as described supra, and is shown in Table3.

Comparative Examples D, E, and F

One 15 g aliquot of each Mixture Component prepared in ComparativeExamples A, B, and C was separately heated to 900° C. as described inExamples 3-6, supra. Following heating, cooling and grinding, theresulting powders of the two Mixture Components were again combined inthe molar ratios shown in Table 3.

Luminescence data on the resulting mixtures was obtained as describedsupra, and is shown in Table 3.

TABLE 1 Maximum Exposure 589/610 Reactants (g) Temp. Peak ExampleCaCl₂•2H₂O SrCl₂•6H₂O EuCl₃•6H₂O NH₄F Composition of Product (C.) Ratio1 2.177 11.838 0.44 4.489 Eu_(0.02) Sr_(0.74) Ca_(0.24) F_(2.02-2y)O_(y) 30 2.60 2 4.322  7.839 0.44 4.489 Eu_(0.02) Sr_(0.49) Ca_(0.49)F_(2.02-2y)O_(y)  30 2.58 3 6.527  3.839 0.44 4.489 Eu_(0.02) Sr_(0.24)Ca_(0.74) F_(2.02-2y)O_(y)  30 3.38 4 2.177 11.838 0.44 4.489 Eu_(0.02)Sr_(0.74) Ca_(0.24) F_(2.02-2y)O_(y) 245 4.61 5 4.322  7.839 0.44 4.489Eu_(0.02) Sr_(0.49) Ca_(0.49) F_(2.02-2y)O_(y) 245 3.74 6 6.527  3.8390.44 4.489 Eu_(0.02) Sr_(0.24) Ca_(0.74) F_(2.02-2y)O_(y) 245 4.79 72.177 11.838 0.44 4.489 Eu_(0.02) Sr_(0.74) Ca_(0.24) F_(2.02-2y)O_(y)900 1.52 8 4.322  7.839 0.44 4.489 Eu_(0.02) Sr_(0.49) Ca_(0.49)F_(2.02-2y)O_(y) 900 1.72 9 6.527  3.839 0.44 4.489 Eu_(0.02) Sr_(0.24)Ca_(0.74) F_(2.02-2y)O_(y) 900 1.68

TABLE 2 Reactants CaCl₂•2H₂O (g) SrCl₂•6H₂O (g) EuCl₃•6H₂O (g) NH₄F (g)Composition of Product MC1 0 15.667 0.44 4.489 Eu(0.02) Sr(0.98) F(2.02-2y)O(y) MC2 13.886 0 0.514 4.489 Eu(0.02) Ca(0.98) F(2.02- 2y)O(y))

TABLE 3 Maximum % % Exposure 589/610 Eu_(0.02)Ca_(0.98)F_(2.02-2y)O_(y)Eu_(0.02)Sr_(0.98)F_(2.02-2y)O_(y) Temp. (C.) Ratio CE A 83 17 245 3.13CE B 62 38 245 3.65 CE C 35 65 245 4.26 CE D 83 17 900 2.20 CE E 62 38900 2.60 CE F 35 65 900 2.84

1.-10. (canceled)
 11. A process comprising combining an aqueous solutionof an ammonium fluoride or hydrogen fluoride, or mixtures thereof, withone or more aqueous solutions of the salts of at least two alkalineearth metals, and an aqueous solution of a salt comprising a 3-valentrare earth metal cation, the amount of the rare-earth metal cation beingin the range of 0.5 to 20 mol-% of the molar concentration of the totalalkaline earth metal cation content, thereby forming a reaction mixturefrom which is formed a precipitate of a rare-earth doped solid statesolution of alkaline earth fluorides represented by the formulaRE_(x)(Ca_(a)Sr_(b)Ba_(c))_(1−x)F_(2+x−2y)O_(y) wherein RE represents athree-valent rare-earth element, 0.005≦×≦0.20, and 0≦y≦0.2, a+b+c=1,with the proviso at least two of a, b, and c are not equal to zero; therare-earth doped multi-valent metal fluoride being characterized by anaqueous solubility of less than 0.1 g/100 g of water.
 12. The process ofclaim 11 wherein the amount of the rare-earth metal cation is in therange of 1 to 10 mol-%.
 13. The process of claim 11 wherein the rareearth is Eu⁺³, Er⁺³, or Tb⁺³.
 14. The process of claim 11 wherein thealkaline earth metals are calcium and strontium.
 15. The process ofclaim 14 wherein the mole ratio of calcium to strontium ranges from 3:1to 1:3.
 16. The process of claim 11 wherein the rare earth is Eu⁺³ in anamount in the range of 1 to 10 mol-%, and the alkaline earth metals arecalcium and strontium in the mole ratio range of 3:1 to 1:3.
 17. Theprocess of claim 11 conducted at a temperature between 20° C. and 100°C.
 18. The process of claim 17 conducted at a temperature between 20° C.and 50° C.
 19. The process of claim 11 wherein the one or more aqueoussolutions of salts of alkaline earth metals are characterized by anormality, the normality being less than 0.01N.
 20. The process of claim11 wherein the one or more aqueous solutions of salts of alkaline earthmetals are characterized by a normality, the normality being greaterthan 0.01N.
 21. The process of claim 20 wherein the normality is in therange of 0.1 to 0.8 normal.
 22. A process comprising heating in thepresence of oxygen a composition comprising a rare-earth-dopedsolid-state solution of alkaline earth fluorides represented by thechemical formulaRE_(x)(Ca_(a)Sr_(b)Ba_(c))_(1−x)F_(2+x−2y)O_(y) wherein RE represents athree-valent rare-earth element, 0.005≦×≦0.20, and 0≦y≦0.2, a+b+c=1,with the proviso at least two of a, b, and c are not equal to zero; to amaximum temperature in the range of 200° C. to 1000° C., followed bycooling, with the proviso that the composition so heated has notpreviously been exposed to the maximum temperature.
 23. The process ofclaim 22 wherein one of a, b, or c=0.
 24. The process of claim 23wherein a=0.01 to 0.99, b=0.99 to 0.01, and c=0.
 25. The process ofclaim 24 wherein a=0.25 to 0.75, b=0.75 to 0.25, and c=0.
 26. Theprocess of claim 22 wherein 0.05≦×≦0.15.
 27. The process of claim 22wherein the rare earth is Eu⁺³, Er⁺³, or Tb⁺³.
 28. The process of claim22 wherein a=0.25 to 0.75, b=0.75 to 0.25, and c=0, 0.05≦×≦0.15, and therare earth is Eu⁺³.
 29. The process of claim 22 wherein the rare-earthdoped solid-state solution of alkaline earth fluorides is in the form ofparticles.
 30. The process of claim 29 wherein the particles arenano-sized.
 31. The process of claim 29 wherein the particles aremicro-sized.